Pulmonary arterial hypertension (PAH) is a devastating disease characterised by severe pulmonary arterial remodelling and occlusive pulmonary vascular lesions, leading to right ventricular failure. Recent epidemiological studies report the incidence of the disease is greater in females; depending on the disease classification the female to male ratio can be as great as 4:11.
PAH has a poor prognosis affecting 2-3/million annually; patients with PAH have a 1-, 3-, and 5-year survival rate of 68%, 48%, and 34%, respectively. Survival is not increased significantly on current therapies and there is an urgent need for novel therapeutic approaches.
Heritable PAH is associated with mutations in the gene for BMPR2 which signals through the smad1/ID pathway2. Serotonin also plays a key role in the development of PAH by facilitation PASMC proliferation and contraction. This can be via its synthesis by tryptophan hydroxylase 1 (TPH1)3-5, by entering the cell via the serotonin transporter (SERT) and/or by activation of the 5HT1B receptor3, 6-13.
We have demonstrated that, in three animal models of pulmonary hypertension (PH) where only the females develop PH (the SERT+ mouse10, the mts1-over-expressing mouse14 and the dexfenfluramine-treated mouse15), the development of PH can be serotonin-dependent and dependent on endogenous oestrogen and/or oestrogen metabolism15-17. This suggests that gender, oestrogen and serotonin are interacting risk factors in PH.
MicroRNAs (miRs) are small non-coding RNAs that have a substantial impact on cellular function through repression of translation (either through inhibition of translation or induction of mRNA degradation). MicroRNAs derive from primary RNA transcripts (pri-miRNA) synthesised by RNA pol II, which may be several thousand nucleotides in length. A single pri-miRNA transcript may give rise to more than one active miRNA.
In the nucleus, the Type III RNAse enzyme Drosha processes the pri-miRNA transcript into a precursor miRNA (pre-miRNA) consisting of a stem-loop or hairpin structure, normally around 70 to 100 nucleotides in length. The pre-miRNA is then transported to the cytoplasm, where it is processed further by the RNAse Dicer, removing the loop and yielding a mature double stranded miRNA molecule, having an active “guide” strand (typically 15 to 25 nucleotides in length) hybridised to a wholly or partially complementary “passenger” strand.
The mature double stranded miRNA is then incorporated into the RNA-induced silencing complex, where the guide strand hybridises to a binding site in the target mRNA.
The guide strand may not be completely complementary to the target binding site. However, a region of the guide strand designated the “seed” sequence is usually fully complementary to the corresponding sequence of the target binding site. The seed sequence is typically 2 to 8 nucleotides in length and located at or near (within 1 or two nucleotides of) the 5′ end of the guide strand.
It is believed that single unpaired guide strands may also be capable of being incorporated into RISC. It is also believed that modifications to the passenger strand (e.g. to the sugars, the bases, or the backbone structure) which impede incorporation of the passenger strand into RISC may also increase efficiency of target inhibition by a double stranded miRNA.
Certain miRs have previously been shown to regulate genes that impact on PAH18-20.